Florida is a US state collapsing in on itself. Amid the city streets, quiet suburbs and citrus groves, gateways are opening up into the real, hidden Florida. Virtually the whole of the Sunshine State, from the Keys in the south to the border with Georgia in the north, is underlain by a strange, cavernous underworld. That’s because Florida is built on a vast platform of limestone bedrock. Just tens of metres below ground, a vast underground river system – the Floridan aquifer – channels billions of gallons of groundwater through subterranean passageways. This buried arterial network provides most of Florida’s fresh water. It keeps the grass green, fills the many swimming pools and slakes the thirst of its residents. But the water is also eating away at Florida’s soluble limestone foundations. And what that means is that the land above is sinkhole country.
Sinkholes mostly form by acid-tinged rainwater slowly eating away the limestone and washing soil and sediment into the cavities. The result is a pockmarked landscape of gentle pits and depressions that can flood to give residential estates much desired ornamental lakes. But some sinkholes form with the sudden collapse of the ground above caves and caverns. These ‘cover-collapse’ sinkholes are the deadly threats that lurk in the Florida subsurface. The trigger mechanism for these sinkhole trapdoors is still a mystery. Some are caused by too much water. Hurricanes can dump tonnes of water on the land over a matter of hours, weighing down the soil and collapsing the roofs of caves below. Others snap open from too little water. Drought years or groundwater pumping can reduce pressure in water-filled voids, causing unsupported sides to implode.